Amusing D facts: typesafe variadic arrays are lazy!

[Andrei Alexandrescu]

downs wrote:
> Did you know the following code compiles?
> 
>> module test;
>>
>> import std.stdio;
>>
>> void Assert(bool cond, string delegate()[] dgs...) {
>>   debug if (!cond) {
>>     string str;
>>     foreach (dg; dgs) str ~= dg();
>>     throw new Exception(str);
>>   }
>> }
>>
>> void main() {
>>   Assert(false, "O hai thar! ");
>> }
> 
> 
> It's true! :)

Gosh!!! What's happening over here? I even tried this:

import std.stdio;

void Assert(bool cond, string delegate()[] dgs...) {
     if (!cond) {
         string str;
         foreach (dg; dgs) str ~= dg();
         throw new Exception(str);
     }
}

string fun(string a, string b) {
     writeln("Concatenating...");
     return a ~ b;
}

void main() {
     Assert(true, fun("O hai thar! ", "wyda"));
     Assert(false, fun("O hai thar! ", "wyda"));
}

This example only prints "Concatenatning..." once, meaning that fun is 
also lazified!!!

This is very exciting! The fact that this little anomaly hasn't caused 
trouble is a good sign it could actually replace lazy!


Andrei